The invention relates to a system for producing a flow of liquid or solid xenon, and also to apparatus for, and a method of, cooling a fluid flow, for example of an inert gas such as xenon, to produce a target for the generation of extreme ultra violet (EUV) radiation.
Extreme ultra violet lithography (EUVL) extends the current technology of optical lithography by using wavelengths in the range from 11 to 14 nm, in order to offer a reduction in the size of printable features during the manufacture of integrated circuits. A known method for the generation of EUV radiation for EUVL is the focussing of an electrical discharge or a laser beam onto a target. Targets are typically formed from metal, such as tin or lithium, or from an inert gas, such as xenon. However, the use of a metal target can result in the evaporation of undesirable debris, such as small particles, atoms and ions, from the surface of the target, which debris may be deposited on sensitive optical components used to focus the beam on the target. The use of metal targets also requires relatively complex optical systems to focus the laser beam over the target surface. These problems may be overcome through the use of an inert gas such as xenon as the target material.
In systems that use xenon as the target material for the generation of EUV radiation, a stream of xenon is emitted from a nozzle into a chamber. As the EUV radiation has a poor transmissibility through xenon, it is necessary to employ a pumping system to generate a reduced pressure in the vicinity of the site at which the xenon plasma is to be generated. Furthermore, the distance between the nozzle and the plasma site should be of sufficient magnitude so as to avoid ablation of the nozzle by the laser beam. In practice, the location of this site is dependent on the density of the stream output from the nozzle; as the density of the output stream is increased, so the distance between the nozzle and the plasma site can be increased.
It is an object of at least the preferred embodiment of the invention to provide apparatus to enable the site at which xenon plasma is generated to be sufficiently spaced from the nozzle that nozzle damage is minimised.
One problem associated with the use of xenon as the target material for the generation of EUV radiation lies in the relatively high cost of the material in comparison to metallic targets, due to the occurrence of xenon in atmospheric air in very low concentrations (around 0.087 ppm). It is therefore another object of at least the preferred embodiment of the invention to provide a system that enables xenon to be retrieved from the evacuated chamber and re-circulated for return to the chamber.